By Stephen Sadler
Researchers at the University of Leicester may have solved a long-standing mystery in astrophysics, namely: how do black holes grow so massive? In a recent paper in the journal Notes of the Royal Astronomical Society, Christopher Nixon and Andrew King from the University of Leicester, along with their collaborator Daniel Price from Monash University in Australia, put forward a new theory of black hole growth in which the gluttonous space-time singularities are able to put on weight more than 100 times faster than their conventional diet allows.
Black holes are ubiquitous in the universe, but the real heavyweights reside at the centre of galaxies like our own Milky Way, and can weigh in anywhere up to 10 billion times the mass of the Sun. They grow by a process called accretion, gobbling up gas from a disc around their equator as it gradually loses angular momentum and spirals inwards, like water down a plug hole.
Except that this can’t possibly be correct, as Professor King explains: “These hugely massive black holes were already full-grown when the universe was very young, less than a tenth of its present age.”
“We needed a faster mechanism,” says Chris Nixon, also at Leicester, “so we wondered what would happen if gas came in from different directions.”
The team made a computer simulation of a black hole orbited by two accretion discs at different angles. At the points where the discs collide, the angular momentum keeping the gas in orbit cancels out, and the gas quickly falls into the central regions, either becoming the black hole’s next meal, or recircularising into a smaller disc, depending on how much angular momentum it managed to hold onto (see picture below).
Simulation of a black hole orbited by two accretion discs, after evolving for approximately 10 orbits. The structure of the two original discs can be seen in blue, whereas the white disc near the centre is formed of gas that has collided and fallen in, before restabilising at a smaller radius.
“If two guys ride motorbikes on a Wall of Death and they collide, they lose the centrifugal force holding them to the walls and fall,” says King. The same thing happens to the gas in these discs, and it falls in towards the hole.
The regions at the centre of young galaxies provide the chaotic conditions thought to give rise to such double-disc accretion systems, providing the greedy young black holes with all the gaseous sustainence they need to grow into the giants we observe today.
The group’s paper can be found at: http://www.astro.le.ac.uk/~cjn12/papers/twist.pdf